1. Field of the Invention
The present invention relates to novel polysiloxane water absorbing materials which can be used for biomedical devices, such as, contact lenses and intraocular lenses. These hydrogels can be fashioned into contact lenses that are water absorbing, soft, hydrophilic, flexible, hydrolytically stable and biologically inert. The hydrogels are prepared from the polymerization of an acrylic-capped polysiloxane prepolymer with a bulky polysiloxanylalkyl (meth)acrylate monomer and at least one hydrophilic monomer.
2. Background
Hydrogels have long been a desirable class of material for the preparation of biomedical devices. See, for example, Wichterle, et al U.S. Pat. No. 3,220,960 which discloses hydrogels comprising a hydrated polymer of a hydroxyalkyl acrylate or methacrylate crosslinked with a corresponding diester (poly 2-hydroxyethyl methacrylate, known as poly-HEMA).
Hydrogels are crosslinked polymeric systems that can absorb and retain water. The physical properties of hydrogels can vary widely and are mostly determined by their water content. Since hydrogels exhibit excellent biocompatibility, there has been extensive interest in the use of hydrogels for biomedical devices, especially contact lenses.
In the field of contact lenses, various factors must combine to yield a material that has appropriate characteristics. Oxygen permeability, wettability, material strength and stability are but a few of the factors which must be carefully balanced to achieve a useable contact lens. Since the cornea receives its oxygen supply exclusively from contact with the atmosphere, good oxygen permeability is a critical characteristic for any contact lens material. Wettability also is important in that, if the lens is not sufficiently wettable, it does not remain lubricated and therefore cannot be worn comfortably in the eye. The optimum contact lens would therefore, have both excellent oxygen permeability, and excellent tear fluid wettability.
Polymeric materials that can be polymerized to form a water-free xerogel are known. Xerogels are understood to be (unhydrated) polymers which swell in the presence of water and retain their water content (i.e., they can be hydrated to form hydrogels). It is also known, with respect to hydrogel materials traditionally used to make contact lenses, that as water content of the crosslinked hydrogel polymers increases, so does the oxygen permeability through the lens to the eye and its cornea. However, as the water content of hydrogel contact lenses exceeds 70% water by weight, certain mechanical characteristics are compromised, thus limiting the oxygen permeability practically achievable in such systems. For example, high-water materials tend to exhibit tearing or other breakage as a result of poor tensile strength. What has accordingly been sought is a highly oxygen permeable material that is also durable and highly wettable.
Silicone-containing materials have been pursued toward this end. While they display very good oxygen permeability and durability, most silicone-containing materials are largely hydrophobic and therefore not sufficiently wettable.
As disclosed in U.S. Pat. No. 4,153,641, various hydrophobic silicone-containing prepolymers such as 1,3-bis(methacryloxyalkyl)-polysiloxanes have been modified by some known hydrophilic monomers such as 2-hydroxyethyl methacrylate (HEMA). However, the resultant contact lenses had a low water content level, and tended to be too stiff to be used as a hydrogel (modulus value over 300 g/mm.sup.2).
Therefore, there remains a need for contact lens material having the high oxygen permeability characteristics of a polysiloxane-containing prepolymer, yet having a modulus low enough to be used as a hydrophilic hydrogel formulation. Such a formulation would be particularly advantageous as a contact lens material.